1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for obtaining diagnostic information, for example, of a B-mode image, by using an ultrasonic wave, and in particular to an ultrasonic imaging apparatus wherein a multi-focus function has been improved.
2. Description of the Related Art
Ultrasonic diagnostic apparatuses include an electronic scan type ultrasonic diagnostic apparatus having an ultrasonic probe (i.e. array probe) in which a number of ultrasonic transducer elements are arranged in parallel. An ultrasonic beam is generated from the ultrasonic probe. The generated beam is electronically focused in a subject, thereby to scan the subject. Based on the ultrasonic echo reflected from the subject, a B-mode image or a two-dimensional blood flow image is produced and displayed on a monitor.
In the case where the above electronic scan type ultrasonic diagnostic apparatus employs a linear electronic scan system, a predetermined number of ultrasonic transducer elements are, as one unit, excited to propagate ultrasonic waves. These excited transducer elements are shifted with a pitch corresponding to one transducer element, whereby the ultrasonic beams generated from the transducer are electronically displaced in the horizontal direction to scan the subject.
In order to converge ultrasonic beams, the ultrasonic transducer elements located near the center of the unit and the ultrasonic transducer elements located on both sides of the unit are excited with a time lag. Thus, ultrasonic beams are propagated to the subject. The wave (echo) reflected from the subject is received by the same transducer element from which the wave is propagated, and is converted into an electric signal, or an echo signal. In this case, the echo signals corresponding to the echo waves received by the transducer elements are subjected to delay processing so that the echo waves are converged. Then, the echo information is displayed on a monitor as a tomographic image. A similar process is also performed in the case of a convex-type scan system.
In the case of a sector electronic scan system, ultrasonic transducer elements are excited with a time lag so that ultrasonic waves generated from the transducer elements are deflected in a the shape of a sector. Echo signals corresponding to echo waves reflected from a subject are processed substantially in the same manner as is employed in the linear electronic scan system. The echo signals are delayed and synthesized to produce a B-mode image signal.
For example, the sector scanning technique for obtaining the B-mode image may be either a single-focus sector scan or a multi-focus sector scan. In the single-focus sector scan technique, ultrasonic beams output from a sector scan ultrasonic probe are focused on one point in a subject. The focal point can be moved by changing the degree of delay of the drive pulses supplied to the transducer elements.
In the single-focus sector scan technique, the degree of delay is suitably changed with respect to each ultrasonic raster so that the focal point is formed at a preset depth. Thus, ultrasonic waves are propagated and the focal point is formed at the preset depth. In the receiving step, image data can be obtained from the focal point at the preset depth.
In the multi-focus sector scan technique, a plurality of focal points can be produced in a single frame at different depths. In other words, a focal point is set at a depth F1 by using a first delay rate, and a focal point is set at a depth F2 by using a second rate. Thus, in the step, two transmission focal points can be produced at depths F1 and F2 by using the two delay rates. In the corresponding receiving step, ultrasonic echos focused at depths different from focal depths F1 and F2 can be collected to obtain data. In an image including the focal points at depths F1 and F2, ultrasonic data of one raster can be obtained by the two ultrasonic transmission/reception steps. Namely, when n-stage focal points are produced, a raster can be obtained by n-rates of ultrasonic transmission/reception. Thus, one ultrasonic image can be obtained at a rate of (n).times.(number of rasters).
As described above, according to the conventional ultrasonic diagnostic apparatus, one or more focal points can be produced at one or more desired depths by multi-focus scanning. Thus, the focused area can be diagnosed with the high-resolution image. In this art, the depths of focal points are fixed and selected by a focus selection switch. Consequently, if a diagnosed part of the body or the scope of visual field changes, the fixed focal points mat become unsuitable.
Suppose, for example, that focal points are set at depths F1 and F2 in a region 1 of interest (ROI) for the purpose of diagnosis with high resolution. In fact, since the depth of a focal point is fixed by each focus selection switch, the depth of a focal point near the region 1 of interest is selected, or the position of a probe is adjusted so that the region 1 of interest corresponds to a suitable depth.
When a region 2 of interest at a depth different from the depth of the region 1 of interest is to be observed, previously set focus positions F1 and F2 mat not necessarily be optimal. At this time, the focal point is often displaced from the region 2 of interest, and the focal point needs to be reset. In resetting the focal point, for example, in the region 1 of interest, a suitable depth is located between F1 and F2. In the region 2 of interest, a suitable depth may be located between F3 and F4. Thus, in addition to the resetting of the focal point, readjusting of the position of the probe is required, resulting in troublesome operation.